An electromagnetic wave from mid-infrared to terahertz wave (THz wave) is defined as an electromagnetic wave having a wavelength of 3 μm to 3 mm. Recently, optical devices operated in these wavelength regions have been actively developed.
The lens is the most basic optical device which converge beams and has been essentially used in various optical systems.
As a lens used to converge the mid-infrared and the terahertz wave, there are, for example, a parabolic mirror, a silicon lens, a polymer lens, and the like.
However, as illustrated in FIG. 1, the existing parabolic mirror 10 has a disadvantage in that an optical path is complicated and thus it is difficult to implement a small optical system.
Further, as illustrated in FIG. 2, it is known that the existing spherical and semi-spherical silicon lenses 20 have good transmittance in the mid-infrared and terahertz frequency regions, but there is a disadvantage in that the spherical and semi-spherical silicon lenses 20 are not easily machined due to characteristics of a crystalline silicon material and therefore are expensive and are not easily miniaturized.
Further, the existing spherical or semi-spherical polymer lens has a disadvantage in that high light absorption appears at a specific frequency, that is, the existing polymer lens used in a visible light region has a disadvantage in that since most polymers have an absorption peak in the mid-infrared and terahertz frequency regions, a specific bandwidth is lost.
FIG. 3 illustrates the existing gradient index lens which uses a conductive layer and a dielectric layer.
As illustrated in FIG. 3, a multilayer structure 12 has an array structure in which rectangular holes 14 and 16 are formed. In FIG. 3, a diameter of the left hole 14 is larger than that of the right hole 16 and the left hole 14 takes a shape in which a diameter is reduced toward the left. The array structure in which horizontal and vertical stripes 18 and 20 are formed between the holes is formed. Meanwhile, the multilayer structure 12 used as a substrate is formed of a multi structure having a shape in which a dielectric substance is sandwiched between the two conductive layers and thus is mainly used for focusing of visible light and IR. Further, since the multilayer structure includes the conductive layer, most region of the visible light and the IR is reflected or absorbed and thus a transmitting region is limited.
Therefore, there is a need to develop a new optical device capable of converging beams in the mid-infrared and terahertz regions by solving the disadvantage of the existing lenses as described above, but a lens or a method for manufacturing the lens meeting the conditions have not yet been suggested. Further, the array structure which has a simple structure and may perform effective focusing has not yet been suggested.